1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus of a fuel supply system for an internal combustion engine and, more particularly, to an air-fuel ratio control apparatus for a fuel injection type internal combustion engine.
2. Description of the Prior Art
An air-fuel ratio control apparatus of a fuel supply system for an internal combustion engine has hitherto been known, which has an air valve disposed in an intake passage of the engine upstream of a throttle valve thereof to cooperate with the throttle valve to define an air pressure chamber and being operative to maintain a substantially constant pressure therein, a fuel circuit having at its downstream end a fuel discharge port open to the intake passage and fuel metering means comprising means defining a variable fuel-metering orifice disposed in the fuel circuit and operatively associated with the air valve such that the fuel-flowing section of the fuel-metering orifice is varied in proportion to the air-flowing section of the air valve, and a differential fuel pressure means operative to maintain a substantially constant fuel pressure difference across the fuel-metering orifice during normal engine operation. An example of the publications which disclose the air-fuel ratio control apparatus of the type referred to is Japanese Laid-Open Patent Publication (Pre-Examination Patent Publication) No. 48-83220 (83220/73).
In the air-fuel ratio control apparatus of the type specified above, the differential fuel pressure means is controlled in accordance with the operating conditions of the engine so that the fuel pressure difference across the fuel-metering orfice is varied according to the engine operating conditions to vary the air-fuel ratio of the mixture to be supplied into the engine. The differential fuel pressure means comprises a differential pressure valve disposed in the fuel circuit downstream of the fuel-metering orifice. An electromagnetic means is employed to control the fuel-flowing section of the differential pressure valve in dependence on the engine operating conditions. In the air-fuel ratio control apparatus disclosed in the Japanese publication referred to above, the differential pressure valve comprises a valve seat and a diaphragm cooperative therewith to define a fuel-flowing gap the size of which is controlled by the electromagnetic means operative in response to variation in the engine operating conditions. In the invention disclosed in the Japanese publication referred to, the engine operating conditions are detected simply by an O.sub.2 sensor which detects the oxygen content of the engine exhaust gas. In the case where other means are employed to detect other engine operating parameters, such as an acceleration sensor and deceleration sensor, for example, in addition to the use of the O.sub.2 sensor, an electric circuitry for controlling the electromagnetic means of the differential pressure valve will be complicated in structure and thus very epensive.